Stenus bimaculatus Gyllenhal, 1810
This is locally common species of wetlands and permanently damp woodlands from lowland to low mountain altitudes throughout Europe north to northern Scandinavia and east through Russia and Kazakhstan to Siberia and Mongolia. Here it is generally common across Wales and most of England becoming sparse and sporadic in the West Country and through Scotland. Typical habitats include a range of wetland marginal situations, particularly reed-bed margins, alder carr and floodplains although it is among the most common of our wetland Stenus and so might occur in any situation; we have found them on peat cuttings in Somerset and on the continent they occur on shorelines. Adults occur year-round but are active and most obvious during spring and summer, they generally occur on soil among patchy vegetation in partially shaded situations and although they may occasionally be swept from vegetation they seem to climb stems etc. less than some other species e.g. S. solutus Er. or S. cicindeloides (Schaller) which, in contrast to the present species have the fourth tarsal segment strongly bilobed. Adults are very active and may run across the water surface when disturbed, they are predatory, hunting other insects etc. but are said to be specialist springtail predators. Adults overwinter among marginal moss and litter, under logs and other debris or under loose bark on trunks and stumps etc.
A large and distinctive species recognized in the field by the maculate elytra and bicoloured legs, the smaller S. guttula Muller, P.W.J. differs by lacking a median keel at the base of the first and second (at least) abdominal tergites. 6.0-7.0mm. Body entirely black, each elytron with a central pale spot and legs mostly pale; apex of femora, base of tibiae and tarsi variously darkened. Head densely punctured and only weakly depressed beside the eyes, antennae entirely dark, maxillary palps pale with the apical segment substantially darkened. Pronotum cylindrical and broadest about the middle, punctures dense and strong but mostly discrete. Elytra much broader than pronotum; with prominent shoulders and widened towards the apex, the punctures dense, strong and sometimes confluent, especially towards the base. Abdomen strongly bordered, the basal segments strongly punctured although much less so than those on the elytra, distal segments less strongly so; basal 4 or 4 with a short but distinct basal median keel. All tarsi about as long as the corresponding tibia, the basal segment, especially on the middle and hind tarsi, longer than the terminal segment. All tarsomeres simple i.e. not bilobed.